An input/output (I/0) circuit is a circuit which has a single terminal which functions as both an input and an output. Numerous system applications exist where multiple I/0 circuits utilizing a variety of differing supply voltages are interconnected. One common application of I/0 circuits is in an alarm system which utilizes multiple smoke detector circuits which are typically powered by batteries. Usually, each alarm unit must be capable of both providing an alarm signal and receiving an alarm signal from any of the other units so that all alarms will be enabled if any one alarm is activated. Furthermore, in smoke detector applications, the possibility exists that the communication wire connecting the units will burn and place a ground potential or `short circuit` on the output of one or more units. Without short circuit protection in the form of current limiting, an output drive transistor could dissipate excessive power, thereby damaging the circuit and rendering the circuit inoperative. Therefore, each I/0 circuit must be protected from possible electrical short circuits. Each I/0 circuit must also be able to both provide an alarm signal and sense an alarm from another unit while operating from a variety of power supply voltages. Therefore, close control of the logic level of the input signal for a wide range of power supply voltages is needed. The possibility of an I/0 circuit receiving an alarm signal having a voltage magnitude which is greater than the power supply voltage of the I/0 circuit also exists. A problem with known I/0 circuits which have been implemented in MOS technology is the lack of tight control of input signal logic levels over varying ranges of supply voltages. This typically results from the large variation of an MOS logic inverter trip point with supply voltage variation. Typical I/0 circuits do not have any output current limiting capability other than a resistor connected to an output terminal. Other known I/0 circuits have been implemented exclusively in bipolar process technology. However, analogous CMOS circuits cannot be built in a P-well CMOS process because an equivalent collector electrode cannot be fabricated. Another common characteristic of other I/0 circuits implemented in MOS processes is the requirement of a start-up circuit to obtain an output signal. Start-up circuitry increases the size and complexity of an I/0 circuit.